Index: .editorconfig
===================================================================
--- .editorconfig	(revision 52934)
+++ .editorconfig	(working copy)
@@ -17,6 +17,10 @@
 indent_style = space
 indent_size = 2
 
+[*.json]
+indent_style = space
+indent_size = 4
+
 [*.md]
 trim_trailing_whitespace = false
 
Index: src/wp-includes/class-wp-theme.php
===================================================================
--- src/wp-includes/class-wp-theme.php	(revision 52934)
+++ src/wp-includes/class-wp-theme.php	(working copy)
@@ -338,11 +338,15 @@
 
 		if ( ! $this->template ) {
 			$this->template = $this->stylesheet;
-			if ( ! file_exists( $this->theme_root . '/' . $this->stylesheet . '/index.php' ) ) {
+
+			$index_php_path = $this->theme_root . '/' . $this->stylesheet . '/index.php';
+			$index_html_path = $this->theme_root . '/' . $this->stylesheet . '/templates/index.html';
+
+			if ( ! file_exists( $index_php_path ) &&  ! file_exists( $index_html_path ) ) {
 				$error_message = sprintf(
 					/* translators: 1: index.php, 2: Documentation URL, 3: style.css */
 					__( 'Template is missing. Standalone themes need to have a %1$s template file. <a href="%2$s">Child themes</a> need to have a Template header in the %3$s stylesheet.' ),
-					'<code>index.php</code>',
+					'<code>index.php</code> or <code>templates/index.html</code>',
 					__( 'https://developer.wordpress.org/themes/advanced-topics/child-themes/' ),
 					'<code>style.css</code>'
 				);
Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 52934)
+++ src/wp-includes/theme.php	(working copy)
@@ -857,7 +857,7 @@
 		return true;
 	}
 
-	if ( ! file_exists( get_template_directory() . '/index.php' ) ) {
+	if ( ! file_exists( get_template_directory() . '/index.php' ) && ! file_exists( get_template_directory() . '/templates/index.html' ) ) {
 		// Invalid.
 	} elseif ( ! file_exists( get_template_directory() . '/style.css' ) ) {
 		// Invalid.